The present invention relates to a method for detecting a fire condition in a monitored region, and in particular, it concerns a method for detecting organic and inorganic fuel fires at relatively long ranges and/or for detecting relatively small organic and inorganic fires.
Existing optical flame detectors are limited in their detection range and in their spectral response to either organic fuel flames (such as flames having a CO2 spectral peak) or inorganic fuel flames (such as flames having a H2O spectral peak). One problem in detecting fire conditions, particularly at long detection ranges or for small fires, is a high false alarm rate. The detection range can be increased by increasing system sensitivity, e.g. by appropriately setting an amplification level and/or a threshold level. However, an increase in sensitivity also tends to increase the false alarm rate, which is caused by spurious radiation sources such as: sunlight; artificial light; welding; electrical heaters; ovens; etc., or by other sources of radiation “noise”. Such spurious radiation sources might not be intense enough to activate short-range detectors, but they may be sufficient to activate a detector whose sensitivity has been increased for an increased detection range.
A false alarm can result in a costly discharge of a fire extinguisher. Furthermore, if the fire extinguisher is of the type requiring replacement before it can be reused, the false alarm can serve to effectively disable the fire extinguisher system until the extinguisher system has been replaced.
A number of attempts have been made to increase the range of a fire detector system without substantially increasing the false alarm rate. Some systems utilize two sensors in different spectral ranges, as described in U.S. Pat. Nos.: 3,653,016, 3,665,440, 3,825,754, 3,931,521, 4,639,598 and 4,983,853, whose disclosure is incorporated herein by reference Other systems utilize an AC coupling and a level ratio test, as described in U.S. Pat. No. 4,455,487, whose disclosure is incorporated herein by reference. In another system, the detector examines the frequency characteristics of monitored signals produced by a sensor to distinguish between fire-emitted radiation and spurious radiation.
European patent EP0926647B1 and U.S. Pat. No. 5,373,159, whose disclosure is incorporated herein by reference, describe a triple-IR method for detecting fire condition using three sensors, each sensitive to a different wavelength bands. The first band is sensitive to wavelengths, which include the CO2 emission band, and the second and third bands are sensitive to wavelengths shorter and longer than the CO2 band. This method is reliable in distinguishing between the radiation emitted by flames having hot CO2 content (e.g. hydrocarbon fires) and other environmental and spurious radiation sources. The method allows increased sensitivity and speed of detection while keeping the probability of false alarms low. However, the method only allows detection of flames from organic materials (hydrocarbons) which emit CO2 in the combustion process.
Flames from fuels such as Hydrogen, Hydrazine, Hydroxyl fuels and Ammonia do not emit any CO2 in the combustion process, but they do emit large amounts of hot water vapors. Flames from these sources can be detected using a similar triple-IR method, where the first sensor is sensitive to wavelengths in one of the water IR emission bands (for example at wavelengths around 2.7 microns). The second and third sensors are sensitive to wavelengths shorter and longer than the water emission band. Such a method can enable detection of water vapor emitting flames with great sensitivity and with a low probability of false alarm. However, this method does not allow detection of flames that do not produce enough water vapor to emit a significant spectral peak in the water emission band.
The two methods described above, namely for detection of CO2-emitting flames and water vapor-emitting flames, do not distinguish efficiently between flames that should be detected and other sources of hot CO2 and/or water vapor, such as steam pipes, industrial burners, and ovens. The characteristic which differentiates between flames and such hot gas sources is the hot gas (i.e. fire combustion product) temperature. The temperature of the gas in the combustion process inside the flame is typically much higher than that of a gas emitted from spurious sources such as steam pipes and ovens. The temperature difference between these different sources has implications on source emission spectral characteristics. For higher temperatures, the gas emits radiation with an intensity more concentrated in shorter wavelengths. For lower temperatures, the radiation intensity is more concentrated in longer wavelengths. Unfortunately, the monitored wavelength bands of the three sensors in the methods described hereinabove are not spread far enough across the spectrum to effectively discriminate between flames and spurious hot sources. If these bands are moved further apart from each other, it would be possible to differentiate between flame temperatures and hot vapor/CO2 temperatures; but the reliability of detection of the CO2 or water vapor spectral peaks would be severely compromised.
Some methods exist (e.g. U.S. Pat. Nos. 5,612,676 and 5,311,167, whose disclosure is incorporated herein by reference) for detection of both hydrocarbon flames (such as CO2 emitting flames) and some non-hydrocarbon flames (e.g. Hydrogen or Ammonia). However, these methods are either less sensitive to the detection of a fire or more prone to false alarms than the triple-IR methods noted hereinabove. The tendency to be less sensitive and/or more prone to false alarms is due to the fact that in these methods not enough data is gathered from the sensors to confidently distinguish between flames (having intensity peak at the CO2 or water vapor emission wavebands) and other sources that do not have these spectral peaks.
There is therefore a need for systems and/or method with:                high sensitivity (ability to sense at longer detection ranges and/or more discriminately);        high selectivity between organic and inorganic fires; and        low false alarm/false detections.        